Transport, Magnetic and Vibrational Properties of Chemically Exfoliated Few Layer Graphene

We study the vibrational, magnetic and transport properties of Few Layer Graphene (FLG) using Raman and electron spin resonance spectroscopy and microwave conductivity measurements. FLG samples were produced using wet chemical exfoliation with different post-processing, namely ultrasound treatment, shear mixing, and magnetic stirring. Raman spectroscopy shows a low intensity D mode which attests a high sample quality. The G mode is present at $1580$ cm$^{-1}$ as expected for graphene. The 2D mode consists of 2 components with varying intensities among the different samples. This is assigned to the presence of single and few layer graphene in the samples. ESR spectroscopy shows a main line in all types of materials with a width of about $1$ mT and and a $g$-factor in the range of $2.005-2.010$. Paramagnetic defect centers with a uniaxial $g$-factor anisotropy are identified, which shows that these are related to the local sp$^2$ bonds of the material. All kinds of investigated FLGs have a temperature dependent resistance which is compatible with a small gap semiconductor. The difference in resistance is related to the different grain size of the samples

Precise determination of graphene functionalization by in situ Raman spectroscopy

The verification of a successful covalent functionalization of graphene and related carbon allotropes can easily be carried out by Raman spectroscopy. Nevertheless, the unequivocal assignment and resolution of individual lattice modes associated with the covalent binding of addends was elusive up to now. Here we present an in situ Raman study of a controlled functionalization of potassium intercalated graphite, revealing several new bands appearing in the D-region of the spectrum. The evolution of these bands with increasing degree of functionalization from low to moderate levels provides a basis for the deconvolution of the different components towards quantifying the extent of functionalization. By complementary DFT calculations we were able to identify the vibrational changes in the close proximity of the addend bearing lattice carbon atoms and to assign them to specific Raman modes. The experimental in situ observation of the developing functionalization along with the reoxidation of the intercalated graphite represents an important step towards an improved understanding of the chemistry of graphene

Fundamentale Aspekte der reduktiven Funktionalisierung von Graphen

This thesis solved fundamental aspects regarding the chemistry and analysis of the reductive exfoliation of graphite for the wet chemical production of graphene. Moreover, the presented results comprise a detailed picture of the concomitant covalent functionalization of the graphenide intermediates. The application and improvement of modern characterization techniques for the determination of the properties of the yielded derivatives of graphene has been accomplished successfully, while innovative developments have been achieved novel and unprecedented levels. For a better understanding of the underlying processes leading to the final graphene derivative, a wide set of analytical tools has been applied, adapted and improved. By refining existing characterization techniques – especially Raman spectroscopy – the composition and properties of reductively exfoliated samples were analyzed in detail. This raised the understanding of the covalent functionalization of graphene to a new level which has been carefully worked out within this work.Die vorliegende Arbeit beleuchtet neue fundamentale Aspekte der reduktiven Exfoliierung von Graphit zur nasschemischen Darstellung von Graphen. Dabei wurden die damit verbundenen Prozesse bei der kovalenten Funktionalisierung von Graphenid Intermediaten eingehend analysiert. Die Anwendung moderner Charakterisierungsmethoden für die schlussendliche Bestimmung der finalen Zusammensetzung der erhaltenen Graphen Derivate wurde mittels innovativer Forschung und Entwicklung erfolgreich umgesetzt. Für ein tieferes Verständnis der zugrunde liegenden Mechanismen, die zu funktionalisiertem Graphen führen, wurde ein breites Spektrum an analytischen Instrumenten modifiziert und verbessert. Durch die Weiterentwicklung bereits existierender Charakterisierungsmethoden - insbesondere der Raman Spektroskopie - konnten Zusammensetzung und Eigenschaften reduktiver exfoliierter Proben in einer Qualität untersucht werden, die bis dato einzigartig ist

Degree of functionalisation dependence of individual Raman intensities in covalent graphene derivatives

Covalent functionalisation of graphene is a continuously progressing field of research. The optical properties of such derivatives attract particular attention. In virtually all optical responses, however, an enhancement in peak intensity with increase of sp3 carbon content, and a vanishing of the peak position shift in monolayer compared to few-layer systems, is observed. The understanding of these seemingly connected phenomena is lacking. Here we demonstrate, using Raman spectroscopy and in situ electrostatic doping techniques, that the intensity is directly modulated by an additional contribution from photoluminescent π-conjugated domains surrounded by sp3 carbon regions in graphene monolayers. The findings are further underpinned by a model which correlates the individual Raman mode intensities to the degree of functionalisation. We also show that the position shift in the spectra of solvent-based and powdered functionalised graphene derivatives originates predominantly from the presence of edge-to-edge and edge-to-basal plane interactions and is by large functionalisation independent

Scanning-Raman-Microscopy for the Statistical Analysis of Covalently Functionalized Graphene

We report on the introduction of a systematic method for the quantitative and reliable characterization of covalently functionalized graphene based on Scanning-Raman-Microscopy (SRM). This allows for recording and analyzing several thousands of Raman spectra per sample and straightforward display of various Raman properties and their correlations with each other in histograms or coded 2D-plots. In this way, information about the functionalization efficiency of a given reaction, the reproducibility of the statistical analysis, and the sample homogeneity can be easily deduced. Based on geometric considerations, we were also able to provide, for the first time, a correlation between the mean defect distance of densely packed point defects and the Raman <i>I</i>_D/<i>I</i>_G ratio directly obtained from the statistical analysis. This proved to be the prerequisite for determining the degree of functionalization, termed θ. As model compounds, we have studied a series of arylated graphenes (GPh) for which we have developed new synthetic procedures. Both graphite and graphene grown by chemical vapor deposition (CVD) were used as starting materials. The best route toward GPh consisted of the initial reduction of graphite with a Na/K alloy in 1,2-dimethoxyethane (DME) as it yields the highest overall homogeneity of products reflected in the widths of the Raman <i>I</i>_D/<i>I</i>_G histograms. The Raman results correlate nicely with parallel thermogravimetric analysis (TGA) coupled with mass spectrometry (MS) studies

Basal-Plane Functionalization of Chemically Exfoliated Molybdenum Disulfide by Diazonium Salts

Although transition metal dichalcogenides such as MoS₂ have been recognized as highly potent two-dimensional nanomaterials, general methods to chemically functionalize them are scarce. Herein, we demonstrate a functionalization route that results in organic groups bonded to the MoS₂ surface <i>via</i> covalent C–S bonds. This is based on lithium intercalation, chemical exfoliation and subsequent quenching of the negative charges residing on the MoS₂ by electrophiles such as diazonium salts. Typical degrees of functionalization are 10–20 atom % and are potentially tunable by the choice of intercalation conditions. Significantly, no further defects are introduced, and annealing at 350 °C restores the pristine 2H-MoS₂. We show that, unlike both chemically exfoliated and pristine MoS₂, the functionalized MoS₂ is very well dispersible in anisole, confirming a significant modification of the surface properties by functionalization. DFT calculations show that the grafting of the functional group to the sulfur atoms of (charged) MoS₂ is energetically favorable and that S–C bonds are formed

Solvent-driven electron trapping and mass transport in reduced graphites to access perfect graphene

Herein, we report on a significant discovery, namely, the quantitative discharging of reduced graphite forms, such as graphite intercalation compounds, graphenide dispersions and graphenides deposited on surfaces with the simple solvent benzonitrile. Because of its comparatively low reduction potential, benzonitrile is reduced during this process to the radical anion, which exhibits a red colour and serves as a reporter molecule for the quantitative determination of negative charges on the carbon sheets. Moreover, this discovery reveals a very fundamental physical–chemical phenomenon, namely a quantitative solvent reduction induced and electrostatically driven mass transport of K+ ions from the graphite intercalation compounds into the liquid. The simple treatment of dispersed graphenides suspended on silica substrates with benzonitrile leads to the clean conversion to graphene. This unprecedented procedure represents a rather mild, scalable and inexpensive method for graphene production surpassing previous wet-chemical approaches

Precise determination of graphene functionalization by in situ Raman spectroscopy

The verification of a successful covalent functionalization of graphene and related carbon allotropes can easily be carried out by Raman spectroscopy. Nevertheless, the unequivocal assignment and resolution of individual lattice modes associated with the covalent binding of addends was elusive up to now. Here we present an in situ Raman study of a controlled functionalization of potassium intercalated graphite, revealing several new bands appearing in the D-region of the spectrum. The evolution of these bands with increasing degree of functionalization from low to moderate levels provides a basis for the deconvolution of the different components towards quantifying the extent of functionalization. By complementary DFT calculations we were able to identify the vibrational changes in the close proximity of the addend bearing lattice carbon atoms and to assign them to specific Raman modes. The experimental in situ observation of the developing functionalization along with the reoxidation of the intercalated graphite represents an important step towards an improved understanding of the chemistry of graphene.© The Author(s) 201